The brief appearance of a visual stimulus leads to an improvement in visual perception. It has been hypothesized that the stimulus evokes a shift of attention to its locus. We have studied these visually evoked shifts of attention in monkeys to determine the constraints on the attentional system. When monkeys are slowed in responding, there is no change in the effectiveness of attention; significant speeding for the animals leads to a reduction in the effectiveness of attention. There is also a reduction in the speed of attentional shifts when animals are tested in a visual field that has little structure. When cues and targets appear at the same location in the majority of trials, there is a great facilitation of behavior. When the proportion of these trials is reduced to 20% for humans, the facilitation is totally abolished, but no such change is present with the same manipulation in monkeys. The data show that humans have substantial cognitive control over attentional processes whereas in monkeys, stimulus-driven attention is involuntary and reflexive. Taken together, these studies show that the monkey is an excellent model for studying the neural processes of sensory-driven shifts of attention. We have compared the head and eye movements evoked by stimulation of the superior colliculus with those obtained from the pontine reticular formation. Stimulation of most sites within the colliculus evokes only a saccadic eye movement, although we have recently discovered regions that will evoke head movements in combination with eye movements. In some cases the position of the eye in the orbit influences the eye movement that is evoked. In every case, the stimulation leads to a shift of gaze, the direction in which the animal is looking. In contrast, by stimulating the pons, we can elicit a head movement but never a gaze shift. Each movement of the head is accompanied by a compensatory movement of the eyes, which means that the direction of sight does not change. These data show that the colliculus participates in the coordination of head and eye movements in order to produce gaze shifts. The role of the pons is in the integration of head and eye movements for gaze stabilization and compensatory movements.